Momentum continues to build behind Facebook's Open Compute Project, which capped its fifth Open Compute Summit this week by welcoming a surprise new participant: Microsoft. With its decision to participate in the OCP's open hardware community, the Redmond giant may yet prove there's more than smoke and mirrors to its embrace of open source.

The OCP, a collaboration launched in 2011, aims to drive down data center costs by developing more efficient servers using open source hardware -- what InfoWorld's Eric Knorr described as "building a mainframe out of Legos." Its model for sharing specifications and hardware designs is more traditionally associated with open source software, but as proof of the OCP's effectiveness, Facebook announced it saved $1.2 billion in energy and management costs by using open source products in the last three years.

At Open Compute Summit, Microsoft announced it will contribute the designs for the cloud servers that run services like Bing and Windows Azure, as well as system management source code. In making the announcement, Bill Laing, corporate vice president for cloud and enterprise at Microsoft, noted that Microsoft and Facebook are the only cloud service providers to publicly release their server specifications, thus vaulting Microsoft into the unfamiliar position of being at the forefront of hardware openness, particularly as compared to secretive Amazon.

"My belief is that they are trying to have a voice in a community that they haven't had a voice before," said Patrick Moorhead, founder and president of research firm Moor Insights & Strategy. "Microsoft may be joining the Open Compute Project to better understand the community ahead of Linux and the do-it-yourself mentality spreading to the enterprise market," Moorhead said.

Microsoft also joined up with Linux developers Red Hat and Suse, server makers HP and Dell, and chipmakers AMD and Applied Micro to announce a new OCP project for accelerating the development of ARM-based servers. ARM server chips, seen as a low-power alternative to x86 for various Web and data analytics workloads, are a key part of Facebook's aim to bring down the costs of data centers. While Microsoft didn't commit to developing ARM versions of Windows Server and Hyper-V, it contributed to a specification to help standardize the ARM server platform.

In an effort to further encourage a broader participation by hardware makers, the OCP announced an expansion of its licensing options to include a GPL-like license for open source hardware "that will require anyone who modifies an original design and then sells that design to contribute the modified version back to the foundation."

Microsoft took the industry by surprise when it launched an open source subsidiary nearly two years ago, but its open efforts since then often came more as the result of a push than a leap of faith. While the company no longer holds with Steve Ballmer's view of Linux as "a cancer," InfoWorld's Bill Snyder notes that its open source efforts have stalled far short of full embrace. Snyder sees the retirement of Ballmer as an opportunity for new leadership to "target open source technologies, as well as the developers who create and deploy them."

By joining the OCP, Microsoft gives a stamp of legitimacy not only to the project, but to its own commitment to open source as well.